Longtime FBI director Robert Mueller named as the lead investigator looking into alleged Russian ties to Trump campaign.

The US Department of Justice named a former head of the FBI as special counsel to lead the investigation into alleged Russian meddling in the 2016 election and possible collusion with Donald Trump's team.

The US president stands accused of seeking to stall the politically explosive probe following his shock dismissal of the FBI chief James Comey, and allegations he asked Comey to drop his investigation of a former aide.

Deputy Attorney General Rod Rosenstein tapped Robert Mueller - a widely-respected figure who headed the FBI for the decade after the September 11 attacks - to take over the FBI's probe on Wednesday of "Russian government efforts to influence the 2016 presidential election and related matters."

"Based upon the unique circumstances, the public interest requires me to place this investigation under the authority of a person who exercises a degree of independence from the normal chain of command," Rosenstein said in a statement.

Capping days of political drama in Washington, the move came as pressure mounted in Congress for an independent probe into ties between Trump's campaign and Moscow, which US intelligence chiefs say interfered to tilt the election in the Republican's favour.

A special counsel is empowered to conduct the investigation independent of the Justice Department hierarchy, with a dedicated staff of his choosing. The counsel is not required to consult with or keep informed the attorney general or deputy attorney generals on the course of the probe.

The special counsel is also authorised to prosecute any crimes unearthed by the investigation.

Trump has consistently rejected any suggestion of collusion between his camp and Moscow as "fake news" and complained in a speech on Wednesday that he had been treated "more unfairly" than any US leader in history during his fledgling presidency.

"As I have stated many times, a thorough investigation will confirm what we already know - there was no collusion between my campaign and any foreign entity," Trump said in a statement.

'Any links'

Pressure to place the Russia probe in independent hands intensified this week following reports that Trump pressured Comey to reel back the Russia investigation as it pertained to Michael Flynn, the national security advisor who was fired over concerns about his Russian contacts.

Trump's alleged pressure on Comey - denied by the White House - has exposed the president to accusations of obstructing justice.

Mueller was director of the FBI from 2001 to 2013, a period when he was forced to shake up a huge bureaucracy blamed for missing evidence that could have prevented the September 11, 2001 attacks.

During his tenure he served both Republican and Democratic presidents, and is highly respected by both parties.

Democratic Party leader Nancy Pelosi said, however, the other ongoing investigations are equally important.

"A special prosecutor is the first step, but it cannot be the last. Director Mueller will still be in the chain of command under the Trump-appointed leadership of the Justice Department.

"He cannot take the place of a truly independent, outside commission that is completely free from the Trump administration's meddling. A special prosecutor does not negate the need for vigorous Congressional investigations either," Pelosi said in a statement.